Home / LEV Testing / Guide
LEV testing - positioning
LEV capture is not something the equipment does entirely by itself. The system provides the pull, but whether the contaminant is actually captured depends on where the source sits relative to the hood - and that is often in the hands of the staff. Capture is a shared job, and positioning is the human half of it, which is why it is easy to overlook and important to get right.
The short answer
LEV capture depends on two things working together: the system generating enough pull at the hood, and the source of the contaminant being positioned within the zone where that pull is effective. The equipment provides the first; the staff often control the second - where they hold the work, where they position an adjustable hood, how close the source is to the extraction. Because capture velocity falls off steeply with distance, a source positioned even a little too far from the hood escapes capture however well the system is working. So capture is a shared responsibility: the system's pull and the staff's positioning both have to be right, and neither alone is enough.
Capture is not the equipment's job alone
There is a natural assumption that once an LEV system is installed and working, capturing the contaminant is entirely the equipment's job - the system pulls, and the contaminant is drawn away. But this is only half the picture. The system does provide the pull, but whether that pull actually captures the contaminant depends on where the source is relative to the hood, and that is frequently determined by the staff - by where they position the work, an adjustable hood, or themselves. So capture is not a purely automatic function of the equipment; it is a shared job between the system and the people using it.
This shared nature is easy to overlook precisely because the equipment is the visible, technical part. Attention goes to the hoods, the fan, the ductwork - the hardware that is tested and maintained - and the human contribution to whether capture actually happens can be forgotten. But an LEV can be in perfect working order, generating exactly the pull it should, and still fail to capture the contaminant if the source is positioned outside its effective reach. The equipment provides the capability; the positioning determines whether that capability is used. Recognising that capture is shared - that the staff hold half of it - is the starting point for making sure it actually happens.
Why position matters so much
The reason positioning matters so much comes down to how capture velocity behaves. The pull a hood generates - the air velocity that draws the contaminant in - is strong right at the hood but falls off very steeply as you move away from it, dropping roughly with the square of the distance. So the zone where the hood can actually capture a contaminant is limited: close to the hood the pull is effective, but a little further out it has faded to almost nothing. Capture only happens if the source sits within that effective zone.
This steep falloff is what makes positioning decisive rather than approximate. It is not that being roughly near the hood is good enough and closer is marginally better - it is that there is an effective zone within which capture works and beyond which it largely does not, and the boundary is closer than people expect because the velocity fades so fast. A source positioned even a little too far from the hood - the work held back from it, an adjustable hood not brought in close, the process drifted away from its extraction - can sit outside the effective zone and escape capture entirely, though the system is pulling exactly as designed. Distance is the difference between capture and escape, which is why where the source sits is so important.
The staff often control the distance
The crucial point is that this decisive distance is often in the hands of the staff. Wherever there is any flexibility in how the work is done relative to the hood - an adjustable or movable hood, a task that can be performed nearer to or further from the extraction, a workpiece that can be held in different positions - the staff are effectively deciding whether the source sits in the effective capture zone. Their positioning choices determine the distance, and the distance determines whether capture happens. So the human half of capture is real and consequential: the people doing the work hold the variable that most directly decides whether the contaminant is caught.
This is why capture cannot be guaranteed by the equipment alone, however good it is. A well-designed, well-tested system sets up the potential for capture - it provides a hood generating adequate pull in a defined zone. But realising that potential depends on the source being positioned within the zone, and that is a human action, repeated every time the work is done. If the staff consistently position the source within the effective zone, the capture the system is capable of is achieved. If they do not - working too far from the hood, not bringing an adjustable hood in - the same system fails to capture, not because it is faulty but because the source is out of its reach. The staff's positioning is the hinge on which the system's capability turns into actual capture.
Making the shared job work
Because capture is shared, making it reliable takes attention to both halves. The system half is handled by good design, maintenance and testing - ensuring the equipment generates the pull it should in the zone it should. The human half is handled by awareness: staff understanding that where they position the source matters, knowing to keep it within the hood's effective reach, and being able to recognise when the source has drifted out. When staff grasp that they hold half of capture - that their positioning is not incidental but decisive - they can position the work to keep the source where the system can catch it.
It also helps when the system is designed to support good positioning rather than fight it. A hood positioned where the work naturally happens, an enclosure that keeps the source in the capture zone by its geometry, or a setup that makes the right position the easy position, all reduce the reliance on staff getting the distance right by conscious effort. The best arrangements make correct positioning natural. But wherever positioning is genuinely in the staff's hands, their awareness of its importance is what completes the shared job. Capture needs both the system's pull and the staff's positioning to be right - so a reliable outcome comes from looking after the equipment and helping the people position the source where that equipment can do its work.
The takeaway
LEV capture is a shared job. The system provides the pull at the hood, but whether the contaminant is actually captured depends on the source sitting within the zone where that pull is effective - and because capture velocity falls off steeply with distance, that zone is limited and its boundary closer than people expect. A source positioned even a little too far from the hood escapes capture however well the system is working.
That decisive distance is frequently in the hands of the staff - through where they position the work, an adjustable hood, or themselves - which makes positioning the human half of capture. The equipment provides the capability; the staff's positioning determines whether it is used. So capture needs both halves right: the system's pull, maintained and tested, and the staff's positioning, informed by understanding that it matters. Neither alone is enough - which is why making capture reliable means looking after the equipment and helping the people keep the source where the system can catch it.
Questions
Only half of it. The system provides the pull at the hood, but whether that pull actually captures the contaminant depends on where the source sits relative to the hood - and that is frequently controlled by the staff. So capture is a shared job between the system and the people using it, not a purely automatic function of the equipment.
Because capture velocity - the pull that draws the contaminant in - falls off very steeply with distance from the hood, roughly with the square of it. So there is a limited effective zone within which capture works and beyond which it largely does not, and the boundary is closer than people expect. Position decides capture or escape.
Wherever there is flexibility - an adjustable or movable hood, a task that can be done nearer or further from the extraction, a workpiece held in different positions - the staff decide the distance between the source and the hood, and that distance decides whether the source is in the effective capture zone. Their positioning is decisive.
Yes. A well-designed, well-tested system provides the potential for capture - a hood generating adequate pull in a defined zone. But realising it depends on the source being positioned within that zone. If staff work too far from the hood, the same system fails to capture, not because it is faulty but because the source is out of reach.
By attending to both halves: good design, maintenance and testing for the system's pull, and staff awareness for the positioning - understanding that where they put the source is decisive, keeping it within the hood's reach, and recognising when it has drifted out. It also helps when the setup makes the right position the natural one.
Partly. A hood positioned where the work naturally happens, or an enclosure that keeps the source in the capture zone by its geometry, reduces reliance on staff getting the distance right. The best arrangements make correct positioning natural - but wherever it is genuinely in staff hands, their awareness completes the shared job.
Our thorough examination confirms your system generates the pull it should in the zone it should - the equipment half of capture, so that staff positioning the source within reach is all that is needed for it to work.
